When applying research findings you must consider all aspects of real world use.
There has been a lot of discussion across the ergonomics community about what is the proper posture for using a keyboard tray. The three typical keyboard tray posture alternatives are negative, neutral or positive. Several years ago there was a big rush to condemn the use of an up tilted keyboard tray or positive slope due to the potential for putting the wrists into extreme extension or bending up. An extended wrist has been shown through peer reviewed research to place a large amount of strain on the wrist and includes increasing the intra carpal tunnel pressure which has also been shown to increase the potential for injury including carpal tunnel syndrome. As a result of this finding, many keyboard tray manufacturers removed that capability to move into a positive slope for their keyboard trays. This removal of the feature has impacted many men who typically prefer a slightly reclined posture.
One prominent study that examined keyboard tray postures used subjects sitting in a typical 90/90 posture (upright with 90deg angles at the arms and legs) and changed the keyboard tray angles for the different tests. The issue with this methodology is that when a person sitting in a 90/90 posture is then faced with a positive tilt keyboard tray, extended or tilted up wrists will result. However, if the subject is placed into a slightly reclined posture, the wrists can be kept straight eliminating the problems of the extended wrist as shown in the picture below.